Additives have been commonly used to try to improve the performance of lubricating oils for gasoline and diesel engines. Additives, or additive packages, may be used for a number of purposes, such as to improve detergency, reduce engine wear, stabilize a lubricating oil against heat and oxidation, reduce oil consumption, inhibit corrosion and reduce friction loss. “Dispersants” are used to maintain in suspension, within the oil, insoluble materials formed by oxidation and other mechanisms during the use of the oil, and prevent sludge flocculation and the precipitation of insoluble materials. Another function of the dispersant is to prevent the agglomeration of soot particles, thus reducing increases in the viscosity of the lubricating oil upon use.
Most dispersants in use today are reaction products of (1) a polyalkenyl-substituted mono- or dicarboxylic acid, anhydride or ester (e.g., polyisobutenyl succinic anhydride), also commonly referred to as a carboxylic acid acylating agent; and (2) a nucleophilic reactant (e.g., an amine, alcohol, amino alcohol or polyol). The ratio of mono- or dicarboxylic acid producing moieties per polyalkenyl moieties can be referred to as the “functionality” of the acylating agent. Most commonly, the nucleophilic reactant is an amine. In order to improve dispersant performance, the trend has been to increase the functionality of the dispersant backbone, and ultimately, increase the average number,of amine moieties per dispersant molecule, and the nitrogen content of the dispersant.
With increasing levels of dispersant nitrogen required to disperse the high level of soot found in modem internal combustion engines, particularly modem diesel engines, compatibility with fluoroelastic engine seal materials has become an issue. A number of approaches to reducing the aggressiveness of dispersant nitrogen have been described. Specifically, to attenuate the deterioration of fluoroelastic seals, the nitrogen of nitrogen-containing dispersants has been rendered non-basic by reaction with various “capping agents”. For example, nitrogen-containing dispersants have been borated and reacted reaction with acids, anhydrides or aldehydes. Although such capping agents generally improve seal compatibility, capping frequently reduces the dispersing properties of the dispersant. Further, direct capping of the dispersant oftentimes causes the viscosity of the dispersant to increase dramatically, making it difficult to blend dispersant concentrates without excessive dilution.
U.S. Pat. No. 3,401,117 describes nitrogen-containing dispersants formed by reaction of metal petroleum sulfonate, maleic anhydride and an amine. Although not used as such in the patent, metal petroleum sulfonates can be used as detergents. In said patent, a neutral metal petroleum sulfonate is maleated to allow for reaction with an amine, and subsequently aminated to provide a dispersant. The maleated metal petroleum sulfonate is strictly an intermediate, and the use thereof in a lubricating oil composition is not suggested.
It would be advantageous to provide lubricating oil compositions containing high levels of dispersant nitrogen from uncapped dispersants, that also display improved compatibility with fluoroelatomer engine seal materials. The present inventors have now found that by reacting a lubricating oil detergent with a certain compounds conventionally used to reduce the basisity of the nitrogen of nitrogen-containing dispersants, and using the resulting modified detergent in a lubricating oil composition also containing a basic nitrogen-containing dispersant, the seal compatibility of the lubricating oil composition can be improved without adversely affecting the performance of the detergent or dispersant.